1. Field of the Invention
The present invention relates generally to position measuring systems and particularly to dot matrix impact printers in which it is necessary to very accurately measure the position of a moving print bar.
2. Description of Prior Art
Impact printers employ individual hammers mounted in a hammer bank to impact an ink ribbon against print paper supported by a platen to effect printing. The printing can be formatted in different schemes such as in the dot matrix fashion where the printed characters are comprised of a series of dots made by the hammers impacting on the print paper. The print paper generally is stepwise advanced each time a line of dots is printed. The hammer bank may be shuttled transversely to the print paper to optimize usage of the individual hammers. One example of a currently available dot matrix impact printer is provided by the Hewlett-Packard Model HP 2608S.
Impact dot matrix printers require a means to measure the relative position of the print bar with respect to the print paper in order to provide timing signals to energize the individual hammer at the proper time to place a dot on the print paper. There are several generally accepted techniques to accomplish this function.
One example is provided by U.S. Pat. No. 4,203,678, Nordstrom et al, "Electronic Control Circuit for a High Speed Bidirectional Printer", issued on May 20, 1980. In the high speed printer of this patent the print head is laterally displaced along a print line by a stepper motor and prints characters comprised of a series of dots. An encoder is coupled to the output shaft of the stepper motor to generate count pulses when the stepper motor displaces the print head. The count pulses are then used by the control circuitry to continuously define print head lateral position.
A second technique utilizes a magnetic velocity transducer and magnetic or optical vane or position sensor. The velocity transducer is so mounted that it measures the velocity of the print bar with respect to the main mechanicsm (the platen is part of this main mechanism). The velocity transducer produces an analog voltage proportional to the velocity which is integrated to produce an analog voltage proportional to print bar position with respect to the (platen) main mechanism. This voltage is then processed by an analog to digital (A/D) converter to provide timing signals. The position sensor generates a home or neutral position signal for periodically zeroing the integrator. Several disadvantages are inherent to this technique. It is difficult to produce a velocity transducer with the required linearity, particularly if the print bar motion spans multiple character positions and utilizes a single print element to print several characters. Adjustments are required to compensate for transducer to transducer variations and gain variations in the integrator. The home position sensor must be mechanically and/or electrically adjusted so that its signal corresponds to the print bar neutral or zero position. In addition, magnetic velocity transducers must be precisely aligned when they are mounted and are sensitive to stray magnetic fields which are common in impact printers.
A third technique uses an optical linear encoder mounted between the print bar and the main mechanism. The linear encoder generally consists of a linear bar pattern printed on a glass or plastic strip or a linear bar pattern composed of slots in a metallic strip and an assembly which includes a light source, apperture plate and light sensors. The strip usually is mounted on the moving print bar and the pickup assembly is mounted on the main mechanism. When the print bar is in motion, the encoder produces pulses corresponding to the position of the print bar. Since the encoder output pulse pattern is fixed by the bar pattern on the strip, it is difficult to accommodate changes in correction factors and dot densities. Critical mounting tolerances necessitate precision mechanical alignment during assembly. A shielding requirement also arises as optical encoders are sensitive to contamination by dirt and dust common to printers.
A further technique such as described by U.S. Pat. No. 4,305,674, Velazquaz, "Position Control Means for Data Printer Heads", issued Dec. 15, 1981, utilizes a rotary encoder coupled to the output shaft of a motor. The rotary motion of the motor is converted to a linear translation to drive a print bar through some type of cam or linkage assembly. The printer of this patent utilizes an optical rotary encoder, in conjunction with a light source and detector to produce a signal train from which the position of the printing head can be determined. Since the motor shaft position is related to the print head position by some nonlinear function, the shaft position relationship to printhead position must be mapped. Rotary encoder systems generally require critical mechanical adjustment of the mechanical linkage and/or phasing of the encoder. To prevent deterioration of the print quality periodic readjustment is necessary to compensate for wear of the mechanical linkage. Also, since the output pulse pattern is fixed by the rotary encoder bar pattern, it is difficult to accommodate changes in correction factors and dot densities.